1. Field of the Invention
The present invention relates to a primary radiator which is used for a reflective antenna for satellite broadcasting, and more particularly, to a primary radiator for transmitting circularly polarized electromagnetic waves.
2. Description of the Related Art
FIGS. 6 and 7 illustrate a conventional primary radiator of this kind. FIG. 6 is a cross-sectional view of the primary radiator, and FIG. 7 is a plan view taken from the direction of the horn part of the primary radiator. As shown in these figures, the conventional primary radiator is equipped with a waveguide 10, which has a horn part 10a at one end, a closed surface 10b at the other end, and a circular shape in cross section; a pair of ridges 11 which protrude from the internal surface of this waveguide 10; and a probe 12 which is disposed between the ridges 11 and the closed surface 10b. 
The waveguide 10 is formed by die casting using a metallic material such as zinc or aluminum, and both of the ridges 11 are formed in the waveguide as a single piece. These ridges 11 have a predetermined height, width, and length, and function as a phase-conversion part (90-degree-phase converter) which converts a circularly polarized wave entering the waveguide 10 from the horn part 10a to a linearly polarized wave. As shown in FIG. 7, when setting the plane including the waveguide 10 and both ridges 11 as a reference plane, the probe 12 intersects the reference plane at about 45 degrees, and the distance between the probe 12 and the closed surface 10b is about one quarter of the guide wavelength.
In a primary radiator having such a structure, for example, when receiving a right-handed circularly polarized wave or a left-handed circularly polarized wave which is transmitted from a satellite, the circularly polarized wave is led into the waveguide 10 from the horn part 10a, and is then converted to a linearly polarized wave when passing through the ridges 11 in the waveguide 10. Specifically, since a circularly polarized wave is a rotating polarized wave which is the sum of the vectors of two linearly polarized waves which have a mutual phase difference of 90xc2x0, 90-degree-phase-shifted phases are converted into the same phase and thus are converted into a linearly polarized wave by passing through the ridges 11. Thus, by combining and receiving the linearly polarized waves at the probe 12, it is possible to convert a received signal to an intermediate frequency signal with a conversion circuit, which is not shown in the figure.
However, in a conventional primary radiator having the structure described above, since the horn part 10a which has a desired aperture diameter and length is formed in a single piece at the end of the waveguide 10, and besides, since the ridges which have a predetermined length are formed in a single piece on the internal surface of the waveguide 10, there has been a problem in that the primary radiator becomes long in the axial direction of the waveguide 10. Also, there has been a problem in that when forming such a waveguide 10 by die casting, the ridges 11 which function as a phase-conversion part has an undercut shape, thus the molding die becomes complicated, resulting in an increased cost.
The present invention is made in view of the foregoing, and an object is to provide a primary radiator which is suitable for miniaturization at low cost.
In order to achieve the above object, in the present invention, a primary radiator includes a waveguide, a probe which protrudes from the internal surface of the waveguide towards a central axis thereof, and a dielectric feeder, which is held by the waveguide. The waveguide is closed at one end and open at the other end. The dielectric feeder includes a radiation part which widens from an aperture of the waveguide, a phase-conversion part which has a plate shape and which intersects the probe at an angle of substantially 45 degrees, and an impedance-conversion part which stands between the radiation part and the phase-conversion part which are formed integrally. The impedance-conversion part becomes narrower while arching towards the interior part of the waveguide.
In a primary radiator with this arrangement, when a circularly polarized wave enters the waveguide from the radiation part of a dielectric feeder, the circularly polarized wave is propagated from the radiation part, and through the impedance-conversion part to the phase-conversion part, converted to a linearly polarized wave at the phase-conversion part, and then combined at the probe. At that time, since the impedance-conversion part has a shape which converges and becomes narrower in the direction from the radiation part to the phase-conversion part, it is possible to drastically decrease the reflection component of the electromagnetic wave which is propagated in the dielectric feeder. Besides, even though the length of the part from the impedance-conversion part to the phase-conversion part is shortened, the phase differences for the linearly polarized becomes large, thus the total length of the primary radiator can be shortened drastically. Also, it becomes unnecessary to form integrally the horn part and the ridges (phase-conversion parts), thereby making it possible to simplify the waveguide shape, resulting in decreased cost.
In the structure described above, it is possible to realize an arch shape of the impedance-conversion part by connecting a plurality of very small inclined planes step-wise. However, it is preferable for the impedance-conversion part to have a cross-sectional shape which includes an approximately quadratic curve, and converges. With the impedance-conversion part having such a shape, it is possible to decrease reflection effectively.
Also, in the structure described above, if the phase-conversion part has an end face on the side opposing the closed surface of the waveguide, steps are formed on the end face, and the steps have two reflection surfaces which are spaced from each other by a distance of about one quarter of the guide wavelength, the phases of the electromagnetic wave which is reflected by the two reflection surface of the steps are inverted and cancelled, thus it is also possible to eliminate impedance mismatching at the end face of the phase-conversion part.
Moreover, in the structure described above, if the radiation part has a trumpet-like shape which widens from the aperture of the waveguide, and has an end face on which annular grooves having a depth of about one quarter of the electromagnetic wave are formed, the phases of the electromagnetic wave which is reflected by the end face and the annular grooves of the radiation part are inverted and cancelled, thus impedance mismatching at the end face of the radiation part is eliminated, thereby making it possible to decrease drastically the reflection component of electromagnetic waves incident on the dielectric feeder.